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Shoulder

The shoulder is a complex combination of bones
and joints where many muscles act to provide the widest range of motion of any
part of the body. Numerous muscles help stabilize the three joints of the
shoulder while giving it motion.
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In Depth:Shoulder

The shoulder is a complex combination of bones and joints where many muscles
act to provide the widest range of motion of any part of the
body. Numerous muscles help stabilize the three joints of the shoulder while
giving it motion.

At the shoulder, three major bones meet and create a 90-degree angle:

Clavicle:
Also known as the collarbone, the clavicle extends across
the front of the shoulder from the sternum to the scapula. It helps
stabilize the shoulder’s movements.

Scapula:
More commonly known as the shoulder blade, the scapula is
a flat triangular bone located in the upper back. It connects with the
collarbone at the front of the body.

Humerus: The
largest bone of the arm, the humerus connects to the scapula and clavicle
in the shoulder. The head of the bone has a ball-like knob to create a
ball-and-socket joint with the scapula.

The junctions of these three bones and the sternum (chest bone) form three
joints:

Glenohumeral joint:
This shallow ball-and-socket-style joint created by the humerus and
scapula allows the arm to rotate circularly and to move up and out from
the body. It is surrounded by soft tissue and strengthened by fibrous
ligaments.

Acromioclavicular
(AC) joint: This joint forms the highest point of the shoulder
and provides the ability to raise the arm above the head. The meeting of
the scapula and clavicle forms it.

Sternoclavicular
joint: This joint is at the center of the chest where the
clavicle meets the sternum. It allows the clavicles to move.

Each joint is surrounded by:

cartilage, to pad the meetings of the bones;

ligaments, to connect the bones;

muscles; and

tendons, to attach the muscles to the bones.

The collection of muscles and tendons in the shoulder is known as the rotator
cuff. It stabilizes the shoulder and holds the head of the humerus in
the glenoid, a shallow cavity in the scapula. The muscles of the rotator cuff
include the suprasinatus, infraspinatus, teres minor, and subscapularis.

Because shoulders have such range of movement and are used often, they are
commonly injured. Some injuries include: